Method of making a near infrared absorbing composition



United States Patent Ofiice 3 218 261 METHOD or MAKiNG A NEAR INFRAREDABSORBING COMPOSITION Walter George Gall, Shellbourne, Del., assignor toE. I. du Pont de Nemours and Company, Wilmington, Del., a corporation ofDelaware No Drawing. Filed Mar. 6, 1962, Ser. No. 177,726 2 Claims. (Cl.252300) This invention relates to the use of infrared energy absorbersto reduce the heat transmission of constructions transparent to visiblelight.

Conventional vehicle Windshields, rear windows, side glass panels, andceiling glass panels transmit all wave lengths of light about equally.The sun emits electromagnetic radiation over a wide, continuous band ofwavelengths stretching from the ultraviolet to the far infrared region.Because of attenuation by absorption by atmospheric water vapor, carbondioxide and ozone, and because of scattering by atmospheric dustparticles, only a relatively narrow band of wavelengths, from 029p. to2.1,, penetrates to sea level on the earth. The intensity of thisradiation is peaked sharply at 0.50,, so that radiation in the regionfrom 0.29;]. to 0.40 commonly called near ultraviolet comprises only 4%of the total solar energy reaching the earth with the remaining 96%being distributed almost equally between the visible (0.40-0.75 and thenear infrared areas (0.752.1,u). Therefore, it can be seen that byselectively absorbing or reflecting the infrared while transmitting thevisible light in vehicle windows, one can decrease the heating effect ofthe sun on the interior of such vehicles.

It is an object of this invention to provide a family of compounds whichabsorb infrared radiation to a greater degree than visible radiation.

This invention has as a further object the provision of an infraredabsorber which can be incorporated into the interlayer of safety-glass,which may be any polyvinyl acetal, although polyvinyl butyral ispreferable.

These objects are accomplished by the following invention in which aniron cyanide complex is incorporated in the polyvinyl butyral interlayerof safety-glass at a concentration of 0.01% to 5% by weight, based onthe interlayer.

In the past, near infrared has been absorbed by transition metal ions,which are only weakly absorptive in the near infrared. Of these, theferrous ion is highly selective, exhibiting little absorption of visibleradiation. Two types of glass; namely, Solex (Pittsburgh Plate GlassCo.), and EZ-I (Libbey-Owens-Ford Glass Co.), contain metallic ions forthe purpose of absorbing infrared radiation. When these types of glassare used to make standard Windshields using two 120 mil glass layers andone 15 mil polyvinyl butyral interlayer, the resulting windshieldtransmits 69% of the incident visible radiation and 25% of the incidentinfrared radiation. These materials absorb radiation having wave lengthsin the range of 0.70- 1.47 While these additives are suitable for theglass layer, they cannot be used in the interlayer which, being only thethickness of the glass, requires absorbers that are more etficient. Inaddition, these materials interact with the interlayer to the detrimentof certain of its physical properties.

The invention involves the discovery of a large class of unusualcyanide-water complexes of iron which efficiently absorb infraredradiation over a broad range (0.75- 1.47 1.) and which can be dispersedin polyvinyl butyral without disadvantageous side effects. Theseabsorbers are fairly selective and transmit only one-half to one-thirdas much infrared radiation as visible solar radiation, when quantitiesof ODS-0.2% by weight are added to polyvinyl butyral interlayers forsafety-glass.

It has further been found that partial replacement of the complexed ironby manganese, ruthenium or cobalt 3,218,261 Patented Nov. 16, 1965yields products which are either soluble or of much smaller particlesize and, therefore, yield more nearly haze-free mixtures with theinterlayer. Some sacrifice in spectral properties results from thissubstitution, principally due to an increase in the transmission ratioof infrared radiation/ visible radiation.

These iron cyanide complexes are subject to some deterioration over longtime exposures to solar ultraviolet radiation, and this may be inhibitedby the incorporation of known ultraviolet screens.

The infrared absorbers of this invention are preferably not employedover the entire automotive windshield area because they also reduce thetransmission of visible radiation to a greater extent than is desirable.The absorbers of this invention are particularly well suited for highuse in graduated bands in Windshields, or over the entire area of side,rear and overhead glass areas where high transmission of visibleradiation is not so critical. There are, of course, many otherapplications for these absorbers where it is necessary or desirable toreduce the amount of heat associated with the transmission of light.Such applications include architectural uses (windows, skylights),welders, goggles, sunglasses and sheets toplace between the light sourceand the -film in movie and slide projectors. Such infrared absorbersalso can be used to improve the efficiency of devices intended for thetrapping and storage of solar radiation and for the conversion of solarenergy to other forms of energy.

Since organic infrared absorbers have been found to be unstable, and,since salts of transition metals absorb infrared weakly, it was foundnecessary to develop an absorber which functioned by means of anelectron transfer mechanism. Only this latter type of absorber has beenfound to be efficient enough to be useful in practical applications. Theabsorber used in this invention is of the electron transfer type and isan iron cyanide derived from Prussian Blue (ferric ferrocyanide).Prussian Blue has an absorption band principally in the visible region,but by adjusting the ion ratio of FE+++/FE++, it has been found possibleto shift the absorption range into the infrared region.

The basic infrared absorber of this invention can be formed by mixingequimolar portions of an alkali metal ferricyanide and a ferric salt,which yields a red-brown solution apparently containing ferricferricyanide,

Upon standing or heating of this solution, a green color is developedand a precipitate is formed. The precipitate is filtered off, washed anddried in a vacuum oven producing products ranging in color from blue togreen, depending on the concentration of reactants and the heating timeand temperature.

Although the above-formed product is insoluble in polyvinyl butyral, itis readily dispersed, for example, upon milling with the polyvinylbutyral. Such a procedure yields a blue mixture which exhibits some redhaze. A glass laminate containing a 15 mil film containing 0.1% byweight of the absorber has a strong infrared absorption maximum at 1000m (6% transmis sion), and accompanied by a high visible transmissionmaximum at 500 m (54% transmission). What is even more unprecedented isthe infrared absorption bandwidth of 880 III/1., completely covering theentire area of high solar intensity. The breadth of this absorption bandaverages 900 mg for other preparations of the invention, as disclosed inthe following examples. This breadth is about six times that of ordinaryelectronic absorption bands. The average spectral transmission for a0.1% by weight concentration of the iron cyanide complex of thisinvention in a 15 mil polyvinyl butyral safety-glass laminate is 19.8%transmission of total solar radiation, 27.6% transmission of visibleradiation, and 11.5% transmission of the near infrared radiation. For asimilar laminate, containing 0.2% of the complex in the 15 mil polyvinylbutyral layer of the foregoing, transmission percentages are 9.8% oftotal solar radiation, 14.7% of and ferricyanide are 0.5-1.0 molar. Ifthe concentration of the reactants is below 0.2 molar, the product has alower wavelength absorption maximum (900 m and a narrower bandwidth (730m than the preferred the visible radiation, and 4.5% of the nearinfrared 5 product. radiation. In addition, such a product is olivedrab, rather than A superior product is obtained if the mixtureresultthe thermochromic blue-green of the preferred product. ing fromthe addition of ferric salt to ferricyanide Other ferric salts, such asferric nitrate, may be subsolution is heated rather than allowed toconvert to the stituted for the preferred ferric chloride as may be seengreen form at room temperature. The infrared absorber from the followingexamples. is fully developed after 30 minutes at 90 C. Longer Thepreviously discussed infrared absorbers are inperiods produce littleadditional improvement. The mole soluble in polyvinyl acetals, includingpolyvinyl butyral, ratio of ferric salt/ferricyanide determines theproduct but are readily dispersible by milling with the polyvinyl thatis obtained. At a mole ratio of 0.9-1.1, the preferred acetal. Suchdispersions generally produce a red haze. blue-green product isobtained. This product is thermo- It is possible to productmodifications of these absorbers chromic and is green at roomtemperature and blue at which are relatively haze-free in polyvinylbutyral. These temperatures above about 50 C. When dispersed in arelatively haze-free absorbers are preferred in many applipolyvinylacetal, it is generally blue at room temperature. cations. Theunmodified absorbers have infrared absorp- At a mole ratio of 0.80.9, apurple infrared absorber tion characteristics that are slightly moredesirable than is produced which is not thermochromic. At a mole ratiothose of the relatively haze-free type and are preferred of 1.2 a blueproduct is produced which is not thermoin many applications where redhaze is not objectionable. chromic and which is still a good absorber inthe near One way of producing a low-haze infrared absorber infraredregion. At a mole ratio of 1.33, a purple solid is to add about 10% byweight of concentrated HCl to is obtained which is not thermochromic andwhich is not the ferric salt solution, and then proceed as described aninfrared absorber. above in the preparation of the basic infraredabsorbers.

The preferred product is obtained by adding the ferri- This solubleproduct is less Weather-resistant and has cyanide solution to the ferricsalt solution at room temabsorption properties considerably inferior tothe disperature. If the solutions are heated to 55 C. before miX-persible product. ing and then heated for one hour at 90 C., aftermixing, A preferred method for preparing low-haze infrared a greenproduct which is not thermochromic is obtained. absorbers is to modifythe above-described iron cyanide- Although such a product forms a bluedispersion in polywater complexes with cobalt, ruthenium or manganese.vinyl butyral, it is different from the preferred product Thesemodifiers conveniently are incorporated into the in having a shorterwavelength infrared absorption maxiferric salt solution by adding CoClRuCl or MnCl mum (820 III/L), and a narrower absorption bandwidth Saltsother than the chlorides may be used, but the (7 20 m chlorides aregenerally most convenient.

Solutions of the reactants which are mixed at room Specific examples forforming the infrared absorbers temperature and then heated at 90 C. forlonger periods of this invention are given in Table I. In Table I all(up to 17 hours) also yield the preferred absorbers. light transmissionfigures are based on 0.1% by weight However, if these solutions arerefluxed for 4 hours or of the absorber product dispersed or dissolvedin a 15 longer, a blue product which is not thermochromic is obmilsafety-glass interlayer of polyvinyl butyral. All contained. Thisproduct has about the same infrared abcentrations are given inmillimoles of the, components of sorption characteristics as does thepreferred product. moderately concentrated aqueous solutions(approximate- The preferred aqueous concentrations of ferric salt ly onemolar concentration), unless otherwise stated.

Table I Solution A Product Example added to Solution B, MillimolesHeating Conditions Yield, No. Solution B, Grams Millirnoles 1OKaFe(CN)010FeC13,TraceK2SzOi lhnsteam bath 20K3Fe(CN) 20 FeCh 1hr roomtemperature. 1.4 10 K1Fe(oN)t bath 2. 9 10 KaFe,(CN)a 1. 1 10K3FB(CN)d--- a. 0 10 K Fe(ON)u 10 Fe(NI-I4)(SO4)1 d 0.05 101M013 10 KFe(ON)R -..dn 2.8 10 K3Fe(CN) 1O FeCh Mixed warm 1hr. steam bath 1.25 10FeCl 10 K Fe(CN)G 17 hrs. steam bath 2,7 10 FeCl; 10 K3Fe(CN)e 15 min.steam bath 2.3 K3Fe(ON)n 40 Fe(NH4)(SO4)2 Mixed warm 4 hrs. steam bath6.4 20 K Fe(ON) 5.5 hrs. reflux 5,2 20 K3FQ(CN)6 h 7 20 K3Fe(CN)e. 5. 410 K3F8(CN)5 1. 9 10 K Fe(ON)0 2. 5 10 K3Fe(CN)t 9 F6013, 0.5 C0013 10K3F6(CN)1... 9 QC13,[C0(NH35(H2O)]C 2.5 10 K3Fe(CN)5. 9 e013, 0.5 RuCla2.8 10 K Fe(CN)t- 9 3.1 10 KaFe(CN)0- 9 1.7 10 K3FG(ON)0. 9. 2.710KiFe(ON)B 9 2.4 10K3Fe(CN)0 9 2.3 10 K3Fe(CN)u. 8 1.9 10 K3Fe(CN)a 92.4 10 KaFe(CN)s 7 2.5 10 KaFe(CN)a-. 6 1.5 10 K3Fe(CN)e.-. 2.8 10KaFe(CN)e, 2. 7

15 ml. Hi0. 10K Fe(CN)@, 8FeCl;1,2MnOlz,40ml.HzO do .5

40 ml. H20. 10 K Fe(CN).-,, s M013, 2 [Co(NH5)5C1]Clz --do 2. 4

25 I111. H20. 10 K3Fe(CN)u, 9.7 E5013, 0.3 COFz, 25 ml. H1O -.do 2. 4

25 m1. H20. 10 K3Fe(CN)t, 8.7 FeCl 1.3 MuCh, 10 m1. H2O 1 hr. reflux 2.6

Table I-Contmued Peak Visible Infrared Minimum Solar Transmission HazeLevel 1 Transmission Transmission Absorption Example Product Color inPolyvinyl Bandwidth,

No. Butyral m l m Percent m Percent Percent Percent Percent Trans.Trans. Visible Infrared Total 500 56 880 10. 5 570-1, 400 31. 6 15. 823. 9 500 64 1, 000 23. 5 580-1, 510 43. 8 25. 6 35. 500 54. 1, 060 8560-1, 470 28. 3 12. 0 20.4 500 62. 5 l, 200 18 590-1, 490 41. 9 23. 833. 1 500 52. 5 1, 150 8 570-1, 500 27. 6 11. 5 19.8 500 65 970 10. 5570-1, 240 36. 6 17. 8 27. 5 Blue-green 500 62. 5 1,050 11 570-1, 470Green 490 63 820 7 57 0-1, 310 32. 9 11. 3 23. 8 Blue-green 490 63. 5 1,000 8. 5 560-1, 400 Green 500 54 1,000 6 570-1, 450 Light green d0 49067 1, 000 19 580-1, 380 44. 8 26 0 35. 6 460 43 820 30 610-1, 260 470 50740 12 57 0-1, 000 480 60. 5 740 1. 7 560-1, 090 27. 3 20.8 24. 1 490 55790 1. 5 560-1, 150 22. 8 15. 4 19. 2 500 55. 5 900 6 570-1, 380 470 53750 1. 5 560-1, 130 23. 7 l9. 6 21. 7 500 48 900 4 560-1, 420 23. 1 9. 616. 6 490 51. 5 900 5 560-1, 380 24.7 11. 8 18. 4 490 60 820 6 580-1,150 31. 8 20.3 26. 2 470 57 750 4. 5 560-1, 080 490 62. 5 800 5 570-1,240 31.4 17 9 24 8 Blue-green 480 61. 5 780 2. 5 560-1, 220 Green 500 45800 0 560-1, 280 18. 0 11. 3 14. 7 Blue-green 500 52. 5 900 5 580-1, 33027. 4 14. 0 20. 9 480 56 750 1. 5 560-1, 230 24. 6 16. 3 20. 6 490 54830 4. 5 570-1, 250 27. 4 17. 1 22. 4 500 61 800 6 580-1, 220 33. 3 21.3 27. 5 500 60 1, 050 11. 5 570-1, 470 34. 3 16.4 25. 6 500 51. 5 880 357 0-1, 320 24.4 12. 3 18. 5 510 50 1, 020 7. 5 590-1, 450 29. 1 13. 321.4 510 46 850 6. 5 590-1, 300 27. 6 15.0 21. 4 33 do 480 60 810 5560-1, 250 29.3 17.4 23. I 34 Bluish purple- 480 59. 5 880 6 550-1, 31028.1 15. 8 22. 5

green.

107 As measured by Gardiner Hazemeter. High indicates a value greaterthan approximately while Low" is less than As can be seen from theexamples, the unmodified complexes generally have superior infraredproperties. The manganese-modified product was considered the best ofthe low haze absorbers. The preferred range of manganese is from 1 to 40atom percent, based on total metal content, with to atom percent beingespecially preferred. Manganese-modified absorbers are preferredbecause, upon weathering, their transmission of visible radiationimproves (about +12%) with little change (about 1.5%) in their infraredabsorption. On the other hand, all the absorbers which were tested forweatherability deteriorated to at least a small degree, as judged fromthe fact that the difference between their infrared transmission andtheir visible transmission became smaller after weathering. Resistanceto weathering can be improved by incorporating an ultraviolet lightabsorber. This can be coated on the polyvinyl butyral sheet orincorporated in the sheet along with the infrared absorber. Stillanother alternative may be employed if two interlayers are to beemployed in a single safety-glass laminate; namely by incorporating theultraviolet absorber in one interlayer while incorporating the infraredabsorber in the other interlayer. Suitable ultraviolet absorbers areTinuvin P, Cyanosorb UV-24 or nickel dibutyldithiocarbamate atconcentrations of approximately 12% by weight of the interpolymer.Tinuvin P has the chemical structure @ibm.

Nickel dibutyldithiocarbamate has the chemical structure Safety-glasspanels which contain the infrared absorbers of this invention in thepolyvinyl butyralinterlayer are formed in the conventional way and havestructural properties similar to safety-glass which contains no infraredabsorber.

Analysis of the preferred, unmodified iron complex infrared absorbersyields values in the range while the cobalt, ruthenium and manganesemodified iron cyanide complexes lie in the range where x may have avalue between 0 and 0.4 and M is Mn, Ru or C0. The very best absorbershave 5.2 to 5.5 cyanide groups per two metal atoms.

I claim:

1. A method of making a near infrared absorbing composition whichcomprises the steps of mixing an aqueous solution of an alkali metalferricyanide with an aqueous solution of a ferric salt, said solutionsbeing about from 0.5 to 1.0 molar, and the mole ratio of ferricsalt/ferricyanide being from 0.8 to 1.2, heating the thus formedsolution at from about C. to the reflux temperature, for from when aprecipitate is formed up to about 17 hours, separating the thus formedprecipitate from the aqueous phase by filtration, and drying saidprecipitate to obtain the near infrared absorbing composition.

2. A method of making a near infrared absorbing composition whichcomprises the steps of mixing an aqueous solution of an alkali metalferricyanide with an aqueous solution of a ferric salt and up to 40 molepercent based on the moles of alkali metal ferricy-anide present of asalt of an element selected from the group consisting of cobalt,ruthenium, and manganese, said solutions being about from 0.5 to 1.0molar, and the mole ratio of said salts/ferricyanide being from 0.8 to1.2, heating the thus formed solution at from about 90 C. to the refluxtemperature, for from when a precipitate is formed up to about 17 hours,separating the thus formed precipitate from the aqueous phase byfiltration, and drying said precipitate to obtain the near infraredabsorbing composition.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCESHackhs Chemical Dictionary, Second edition, Blakistons Son & Co. Inc.,Philadelphia, Pennsylvania, 1937, pages 761 and 966.

JULIUS GREENWALD, Primary Examiner.

1. A METHOD OF MAKING A NEAR INFRARED ABSORBING COMPOSITION WHICHCOMPRISES THE STEPS OF MIXING AN AQUEOUS SOLUTION OF AN ALKALI METALFERRICYANIDE WITH AN AQUEOUS SOLUTION OF A FERRIC SALT, SAID SOLUTIONSBEING ABOUT FROM 0.5 TO 1.0 MOLAR, AND THE MOLE RATIO OF FERRICSALT/FERRIC BEING FROM 0.8 TO 1.2, HEATING THE THUS FORMED SOLUTION ATFROM ABOUT 90*C. TO THE REFLUX TEMPERATURE, FOR FROM WHEN A PRECIPITATEIS FORMED UP TO ABOUT 17 HOURS, SEPARATING THE THUS FORMED PRECIPITATEFROM THE AQUEOUS PHASE BY FILTRATION, AND DRYING SAID PRECIPITATE TOOBTAIN THE NEAR INFRARED ABSORBING COMPOSITION.